Method for coating a moving steel strip with a metal or metal alloy coating

ABSTRACT

A method for coating a moving steel strip with a metal or metal alloy coating, wherein the steel strip runs through a bath of molten metal or metal alloy to coat the steel strip, and the coated steel strip is wiped to control the thickness of the coating using foils or sheets. The bath of molten metal has a volume at most 10,000 times the volume V of the coating on the steel strip per second, volume V in m3 being given by the formula V=2×d×w×s, wherein d=the thickness of the coating in meters, w=the width of the strip in meters, and s=the velocity of the strip in meters per second.

The invention relates to a method for coating a moving steel strip with a metal or metal alloy coating, wherein the steel strip runs through a bath of molten metal or metal alloy to coat the steel strip, and the coated steel strip is wiped to control the thickness of the coating.

Such a method is well known in the art and is called hot dip coating. Usually hot dip coating is performed using a container with a molten metal bath, through which the steel strip is guided using a submerged guiding roll. Normally the strip is forwarded from an annealing line and enters the metal bath through a snout under an angle at an elevated temperature. After rounding the guiding roll, the steel strip leaves the metal bath in a vertical direction, on both sides coated with a metal layer. For commercial use this metal coating is too thick, and a wiping device is present not far above the metal bath to wipe off the surplus of metal. The metal normally is zinc, a zinc alloy, aluminum or an aluminum alloy, but the use of other metals is also possible. Usually air knives are used to wipe the metal from the steel strip. Since the steel strip can be up to 2 meters in width, the metal bath often has a volume of about 24 cubic meters.

Recently a novel type of wiping device has been developed, as described in Belgian patent application 1018202. This wiping device mainly consists of two foils or sheets which are pressed towards the steel strip using a number of springs. This wiping device is placed above for instance a zinc bath as described hereinabove in the place of the usual air knives. When the zinc coated steel strip moves through a gap between the foils of the wiping device, the foils are more or less planing, floating or gliding on the liquid zinc on the steel strip, similar to the well-known aquaplaning effect. By adjusting the pressure exerted by the springs in relation to the speed of the strip, the thickness of the zinc coating leaving the wiping device can be controlled. In this way coatings with a reduced thickness as compared to coatings formed using air knives can be produced.

However, it has been found that the foil wiping device of BE 1018202 can not be used in practice because zinc dross particles that are formed in and on the zinc bath are entrained with the zinc that forms the coating on the steel strip. Zinc dross particles contain iron and aluminum and are quite hard. The zinc dross particles get stuck between the foil of the wiping device and the steel strip and cause scratches on the steel strip. For this reason, the wiping device of BE 1018202 is not used in practice.

It is an object of the invention to provide a method for coating moving steel strip material with a metal or metal alloy coating by which can coat strip material with a higher velocity of the strip.

It is another object of the invention to provide a method for coating moving steel strip material with a metal or metal alloy coating which can produce metal coatings on steel strip which are reduced in thickness as compared to the existing commercial apparatus.

It is a further object of the invention to provide a method for coating moving steel strip material with a metal or metal alloy coating that is improved as compared to existing commercial hot dip coating.

It is a still further object of the invention to provide a method for coating moving steel strip material with a metal or metal alloy coating that is easier to use than existing commercial hot dip coating.

It is also an object of the invention to provide a method for coating moving steel strip material with a metal or metal alloy coating that is less costly in use than existing commercial hot dip coating.

According to the invention, one or more of these objects are reached with a method for coating a moving steel strip with a metal or metal alloy coating, wherein the steel strip runs through a bath of molten metal or metal alloy to coat the steel strip, and the coated steel strip is wiped to control the thickness of the coating using foils or sheets, wherein the bath of molten metal has a volume that is at most 10,000 times the volume V of the coating on the steel strip per second, volume V in m³ being given by the formula V=2×d×w×s, wherein

d=the thickness of the coating in meters

w=the width of the strip in meters

s=the velocity of the strip in meters per second.

Using such a volume of the bath, the dross particles that grow in the bath can remain small enough not to stick between the steel strip and the foils or sheets of the wiping device. The volume of the bath to be used will at least depend on the coating metal or metal alloy, the growing rate of the dross particles, the coating thickness on the steel sheet and the velocity of the steel strip. For this reason, the volume of the bath is dependant on these last parameters. In this way, the known foil wiper as discussed above can be used, which provides the possibility of a high strip velocity and thin coatings.

A relatively small bath has also the advantage that the coating system using the bath according to the invention is much more flexible than the known hot dip coating, because it is not necessary to keep a large volume of metal at a high temperature anymore, and because there are less or no metal losses due to the forming of metallic dross on the surface of the metal bath.

According to a preferred embodiment, the bath of molten metal has a volume that is at most 2000 times the volume V of the coating on the steel strip per second, preferably a volume that is at most 500 times the volume of the coating on the steel strip per second, more preferably a volume that is at most 100 times the volume of the coating on the steel strip per second. The smaller the volume of the bath, the easier it is to keep the oxidic or metallic dross particles at a size such that they do not stick between the steel strip and the foils or sheets of the wiping device. A smaller bath also has the advantage that a change of coating material on the steel strip can be executed faster.

Preferably the bath of molten metal has a volume such that metallic dross particles formed in the bath of molten metal or metal alloy on average are smaller than the thickness of the metal or metal alloy coating on the steel strip after wiping using foils or sheets. Since the bath of molten metal or metal alloy thus should contain only small metallic dross particles, in the range of a few micrometers, the bath will have a preferred volume in the order of a few liters. The small metallic dross particles cannot be entrapped between the steel strip and a foil or sheet for wiping the metal or metal alloy coated steel, nor cause scratches.

Preferably the wiping of the molten metal or metal alloy is performed using foils or sheets which are pressed towards the moving steel strip, wherein the moving of the steel strip causes a hydrodynamic lifting force by the coating which is in equilibrium with the pressure exerted on the foils or sheets. This is the way the foils or sheets provide the coating according to BE 1018202.

According to a preferred embodiment the bath of molten metal or metal alloy is positioned under the foils or sheets. The steel sheet thus enters the bath from below, which means that for instance in case of emergency the foil wiping device can be opened and the bath can be emptied, without contaminating the steel strip. A change of width or thickness of the steel strip, for which the foil wiping device has to be opened, can be performed without emptying the bath. It is also possible to have the steel strip enter the bath from above, but this does not give this advantage.

Preferably the steel strip is moved vertical or under an angle of at most 45 degrees with the vertical first through the bath of molten metal or metal alloy and than in between the foils or sheets. In this way it is easy to guide the strip.

According to a preferred embodiment the bath of molten metal or metal alloy is in fluid connection with the metal or metal alloy layers between the steel strip and the foils or sheets. This means that the metal or metallic alloy in the bath is in direct contact with the metal or metal alloy layers between the foils or sheets and the steel strip, and thus that no un-wiped coatings are formed before a foil wiping device forms the coatings as these are desired.

According to a preferred embodiment a replenishing bath with metal or metal alloy is used to refill the bath of molten metal or metal alloy. Since the bath through which the steel strip runs is small, this bath should preferably be refilled using a replenishing bath.

Preferably electromagnetic induction is used to retain the molten metal or metal alloy in a container for the bath of molten metal or metal alloy. This is especially important in the situation that the bath is situated under the foils or sheets, but such electromagnetic induction is also useful when the bath is situated above the foils or sheets.

According to a preferred embodiment the metal or metal alloy in the bath is protected from oxygen in the surrounding air. In this way the forming of oxidic dross films is reduced. Preferably, for this a shielding gas is used to protect the molten metal or metal alloy in the bath from outside air.

According to a preferred embodiment the foils or sheets are pressed towards the strip to control the thickness of the metal or metal alloy coating. The pressing can be executed using any one of the usual physical possibilities, such as hydraulically, by using springs, by using pies elements, et cetera.

Preferably the steel strip is annealed before entering the bath of metal or metal alloy. Annealing usually has to be performed after a steel strip is cold rolled, and the annealing also provides a desired surface quality for hot dip coating with a metal or metal alloy coating.

According to a preferred embodiment the steel strip is heated to a predetermined temperature before entering the bath of metal or metal alloy. Especially for third generation high strength steel types it is often necessary to perform an additional heating step just before the steel strip enters the bath with metal or metal alloy coating material.

In a preferred embodiment the temperature of the bath of metal or metal alloy is higher than the temperature of the steel strip when entering the bath, preferably the temperature of the bath being 0-30° C. higher than the temperature of the steel strip, more preferably 10-20° C. When the coating material is zinc or a zinc alloy, the strip entry temperature (SET) preferably is the highest melting temperature of the coating material plus a factor C1, wherein C1 is between 10 and 60° C., and more preferably C1 is between 20 and 40° C. With this value of the SET, the temperature of the bath is SET plus C2, wherein C2 is between 0 and 30° C., and preferably C2 is between 10 and 20° C. In this way the temperature of the steel strip is lower than the temperature of the bath and therefore the forming of dross particles in the bath is retarded.

Preferably the metal or metal alloy coated steel strip is cooled after leaving the bath of metal or metal alloy. Cooling, especially fast cooling, is necessary for third generation high strength steel types, but other steel type must usually be cooled as well after leaving the metal or metal alloy bath.

According to a preferred embodiment the steel strip has a width of 0.75 to 2.25 meters. These widths are economical to coat using the method according to the invention.

Preferably the steel strip has a velocity of 2 to 10 m/s. This velocity is at the high end and higher than the speed that can be used to hot dip coat using air knives.

Preferably the metal or metal alloy coating has a thickness of 1 to 30 μm on each side of the steel strip.

According to a preferred embodiment the metal is zinc, magnesium, aluminum or tin, or an alloy of one of these metals. Especially zinc, magnesium or aluminum are used for hot dip coating, but also alloys thereof such as an alloy of zinc and aluminum or an alloy of zinc and aluminum and magnesium. Tin can also be used, often for very thin coatings.

The invention will be elucidated in the following example.

A steel strip with is moved in a downward direction through a zinc bath and subsequently through a foil wiping device. The zinc bath is contained in a container which forms one apparatus with the foil wiping device, such that the zinc in the zinc bath is directly fed into the zinc layers between the foils and the steel strip. The foils are pressed towards the steel strip such that a coating is formed on the steel strip. A replenishing bath is used to fill the bath with molten zinc, such that the volume in the zinc bath remains substantially constant. The zinc bath will be protected by a shielding gas, such that no oxidation by the outside air takes place.

The zinc layers between the strip and the foils have a velocity zero at the surface of the foils and the velocity of the steel strip at the surface of the steel strip. Thus, the zinc coating formed at the surface of the strip has approximately half the thickness of the zinc layer between a foil and the steel strip.

Dross particles are formed in the zinc bath due to the iron in the steel strip. These dross particles are taken along from the zinc bath with the zinc that is used for coating the steel strip. These dross particles should all be small enough to pass in between the foils and the steel strip. When a dross particle has become too large, it will get stuck between a foil and the steel strip, and cause scratches on the steel strip.

For this reason, the zinc dross particles should have an average size that is smaller than half the thickness of the zinc layer between the foils and the steel strip, which is the same as being smaller than the thickness of the zinc coating leaving the foil wiping device.

The volume V of the coating on the steel strip per second is 2×d×w×s. For a normal operation, the thickness of the coating is 5×10⁻⁶ m (=5 μm), the width is for instance 1 meter, and the velocity of the strip is 2 meter per second. The volume V then is 2×5×10⁻⁶×1×2=2×10 ⁻⁵ m³, this is 0.02 liters or 20 cm³. When a bath is used that is 100 times the volume V of the coating on the strip per second, the bath will thus have a volume of 2 liters.

From the literature (Modeling of iron dissolution during hot dip galvanizing of steel strip, O'Dell, Charles, Vlot and Randle, Material science and technology, 2004) it is known that in traditional zinc bath operation an inhibition layer creation is essential to restrict iron dissolution into the bath because of high residence times, To create an inhibition layer thickness of 100 nm requires about 0.15 second submerging time.

The average residence time of dross particles for a zinc coating thickness of 10 μm then drops from 400.000 seconds in a traditional bath to 125 seconds in the bath having a volume of 0.005 m3. With a dross growth rate of 0.05 μm/s (Thermodynamics and kinetics of alloy formation in galvanized coating, Nai-yong Tang, Zinc Based Steel coating systems production and performance, February 1998, Texas) the dross particle size will be about 6 μm. Thus, the average zinc dross particle size is smaller than the thickness of the zinc coating of 10 μm.

The reduced bath volume makes online bath changes possible. The reduced submerging time enables production of a new class of high strength steel.

It will be clear to the skilled person that other metal coating can also be used, such as aluminium or tin, or an alloy of zinc, aluminium or tin, respectively.

It will also be clear to the skilled person that other process steps can be added to the method, such as annealing before the hot dip coating, and heating before and/or cooling after the hot dip coating.

It will also be clear to the skilled person that the design of the bath according to the invention needs to be such that “dead spots” in the liquid flow are to be avoided. In dead spots, the liquid substantially remains on the same place and is not used for coating the steel strip. In such “dead spots” the residence time of the liquid coating material will become too high and large dross particles could be formed. 

1. Method for coating a moving steel strip with a metal or metal alloy coating, comprising running the steel strip through a bath of molten metal or metal alloy to coat the steel strip, and wiping the coated steel strip to control the thickness of the coating using foils or sheets, wherein the bath of molten metal has a volume at most 10,000 times the volume V of the coating on the steel strip per second, volume V in m3 being given by the formula V=2×d×w×s, wherein d=the thickness of the coating in meters w=the width of the strip in meters s=the velocity of the strip in meters per second.
 2. Method according to claim 1, wherein the bath of molten metal has a volume at most 2000 times the volume V of the coating on the steel strip per second.
 3. Method according to claim 1, wherein the bath of molten metal has a volume such that metallic dross particles formed in the bath of molten metal or metal alloy on average are smaller than the thickness of the metal or metal alloy coating on the steel strip after wiping.
 4. Method according to claim 1, wherein the wiping of the molten metal or metal alloy is performed using foils or sheets which are pressed towards the moving steel strip, wherein the moving of the steel strip causes a hydrodynamic lifting force by the coating which is in equilibrium with the pressure exerted on the foils or sheets.
 5. Method according to claim, wherein the bath of molten metal or metal alloy is positioned under the foils or sheets.
 6. Method according to claim 1, wherein the bath of molten metal or metal alloy is in fluid connection with the metal or metal alloy layers between the steel strip and the foils or sheets.
 7. Method according to claim 1, wherein a replenishing bath with metal or metal alloy is used to refill the bath of molten metal or metal alloy.
 8. Method according to claim 1, wherein electromagnetic induction is used to retain the molten metal or metal alloy in a container for the bath of molten metal or metal alloy.
 9. Method according to claim 1, wherein the metal or metal alloy in the bath is protected from oxygen in surrounding air.
 10. Method according to claim 1, wherein the foils or sheets are pressed towards the strip to control the thickness of the metal or metal alloy coating.
 11. Method according to claim 1, wherein the steel strip is annealed before entering the bath of metal or metal alloy.
 12. Method according to claim 1, wherein the steel strip is heated to a predetermined temperature before entering the bath of metal or metal alloy.
 13. Method according to claim 1, wherein the temperature of the bath of metal or metal alloy is higher than the temperature of the steel strip when entering the bath.
 14. Method according to claim 1, wherein the metal or metal alloy coated steel strip is cooled after leaving the bath of metal or metal alloy.
 15. Method according to claim 1, wherein the steel strip has a width of 0.75 to 2.25 meters.
 16. Method according to claim 1, wherein the steel strip has a velocity of 2 to 10 m/s.
 17. Method according to claim 1, wherein the metal or metal alloy coating has a thickness of 1 to 30 m on each side of the steel strip.
 18. Method according to claim 1, wherein the metal is zinc or aluminum or tin or an alloy of one of these metals.
 19. Method according to claim 1, wherein the bath of molten metal has a volume at most 500 times the volume of the coating on the steel strip per second.
 20. Method according to claim 1, wherein the bath of molten metal has a volume at most 100 times the volume of the coating on the steel strip per second.
 21. Method according to claim 1, wherein the bath of molten metal or metal alloy is positioned under the foils or sheets, and the steel strip is moved vertical or under an angle of at most 45 degrees with the vertical first through the bath of molten metal or metal alloy and then in between the foils or sheets.
 22. Method according to claim 1, wherein a shielding gas is used to protect the molten metal or metal alloy in the bath from outside air.
 23. Method according to claim 1, wherein the temperature of the bath of metal or metal alloy is higher than the temperature of the steel strip when entering the bath, wherein the temperature of the bath is 0-30° C. higher than the temperature of the steel strip.
 24. Method according to claim 1, wherein the temperature of the bath of metal or metal alloy is higher than the temperature of the steel strip when entering the bath, wherein the temperature of the bath is 10-20° C. higher than the temperature of the steel strip. 